Plants and animals respond to MAMPs (microbe-associated molecular patterns). MAMP-activated response pathways are functionally analogous to mammalian TLR-signaling. A high throughput Arabidopsis seedling assay allowed us to make several important discoveries. First, bacterial flagellin (Flg22) elicits the deposition of callose in cotyledons via a pathway that involves the transcription factor MYB51, ethylene signaling, and the synthesis of indole glucosinolates that function as novel secondary messengers. Second, bacterial elongation factor EF-Tu (Efl26) and Flg22 elicit callose production by distinct pathways. Third, Flg22, but not oligogalacturonides (OGs), elicit the production of soluble anti-microbial compound(s). Fourth, the bacterial phytopathogen Pseudomonas syringae blocks MAMP signaling in roots by the production of the jasmonic acid (JA) mimic coronatine. In addition, we have also shown:1) that cell-wall associated peroxidases play a key role in the plant defense response, 2) that infection of Arabidopsis by the opportunistic broad host range pathogen Pseudomonas aerugionsa involves novel cell wall degrading enzymes, and 3) that the insect leaf miner Scaptomyza graminum, which is nested in the Drosophila phylogenetic clade, is an aggressive Arabidopsis herbivore. In the second round of MERIT funding: We will investigate how MAMP-activated pathways relate to defense pathways mediated by salicylic acid, JA and ET. We will determine the molecular mechanism by which coronatine blocks MAMP signaling. Using MYB51 as a model, we will identify the set of proteins required to activate MYB51-responsive promoters. We will identify the soluble antimicrobial factors elicited by MAMPs and elucidate the signaling pathway(s) leading to their synthesis. We will distinguish the roles that peroxidases and NADPH oxidases play in plant defense and use the Scaptomyza model to determine the role of the oxidase burst in insect defense. We will identify P. aeruginosa cell wall degrading enzymes and determine the role(s) they play in plant pathogenesis.